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Abstract
Due to the unique crystal structure, outstanding optoelectronic properties and a tunable band gap from 1.2-1.8 eV, two-dimensional molybdenum disulfide (MoS2) has attracted extensive attention as a promising candidate for future photodetectors. In this work, a negative-capacitance (NC) MoS2 phototransistor is fabricated by using ${\rm Hf}_{0.5}{\rm Zr}_{0.5}{\rm O}_2$ (HZO) as ferroelectric layer and Al2O3 as matching layer, and a low subthreshold swing (SS) of 39 mV/dec and an ultrahigh detectivity of 3.736×1014 cmHz1/2W−1 are achieved at room temperature due to the NC effect of the ferroelectric HZO. Moreover, after sulfur (S) treatment on MoS2, the transistor obtained a lower SS of 33 mV/dec, a detectivity of 1.329×1014 cmHz1/2W−1 and specially a faster response time of 3-4 ms at room temperature, attributed to the modulation of photogating effect induced by S-vacancy passivation in MoS2 by the S treatment. Therefore, the combination of the defect engineering on MoS2 and the NC effect from ferroelectric thin film could provide an effective solution for high-sensitivity phototransistors based on two-dimensional materials in the future.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) materials, such as graphene [1,2] and transition metal dichalcogenides (TMDs) [3–5] have attracted extensive attention due to their unique crystal structure and outstanding optoelectronic properties, which makes them promising candidates for future photodetectors. ${\rm MoS}_2$ is a kind of 2D TMD with high quantum efficiency, high carrier mobility and thickness-depended bandgap (1.2-1.8 eV) [6,7], which is significant for the design and fabrication of high-performance photodetectors from visible to near-infrared range [8–10]. In the recently reported work, the responsivity of ${\rm MoS}_2$ photodetectors has reached 100 A/W and above, but the response time is on an order of 1000 ms [11–13], which is mainly attributed to those defect states in ${\rm MoS}_2$ or at the interface of ${\rm MoS}_2$ and ${\rm SiO}_2$ substrate. How to reduce the defect states in ${\rm MoS}_2$ to obtain a faster response time while improving the responsivity ($R$) and detectivity ($D^*$) has become an urgent event that restricts the development of ${\rm MoS}_2$ photodetectors.
In the optimization strategy of 2D-channel transistor, inserting a ferroelectric thin film in gate stack has become a widely used method [14–16]. As a mature ferroelectric material, hafnium zirconium oxide (${\rm Hf}_{0.5}{\rm Zr}_{0.5}{\rm O}_2$, HZO) has been extensively studied as a ferroelectric layer of gate stack, and its high $k$ value, CMOS process compatibility and environmental friendliness make it promising for future microelectronic devices [17,18]. The negative-capacitance (NC) effect generated by the polarity reversal of ferroelectrics [19] can amplify gate voltage and obtain a steep subthreshold swing ($SS$) below 60 mV/dec [20–23], which can effectively enhance the photogenerated current of the ${\rm MoS}_2$ photodetector while maintaining response speed [20]. The structural defect density of ${\rm MoS}_2$ are much higher than graphene etc [24–26] due to its low atomic formation energy, and the sulfur vacancies (SVs) are a main defect type in ${\rm MoS}_2$ prepared by micromechanical exfoliation or chemical vapor deposition (CVD) [26–28].
In this work, the ferroelectric HZO and ${\rm Al}_2{\rm O}_3$ dielectric were used to form the stacked gate (HZO/${\rm Al}_2{\rm O}_3$) of ${\rm MoS}_2$ phototransistor. The transistor obtained a $SS$ of 39 mV/dec at low gate-source and drain-source voltages of $V_{gs}$ = 0.45 V and $V_{ds}$ = 0.4 V, a responsivity of 579.37 A/W and a detectivity of 3.736×1014 cmHz1/2W−1. To reduce the response time of the MoS2 photodetector, we investigated the influence of S passivation treatment on the performance of the ${\rm MoS}_2$ photodetector, a largely reduced response time of 3-4 ms has been obtained by repairing S vacancy in the MoS2 flake. This work lies in a combination of the defect engineering on ${\rm MoS}_2$ with the negative capacitance effect of ferroelectric HZO film to achieve high detectivity and faster response speed, which can provide an effective solution for high-sensitivity phototransistors based on two-dimensional materials in the future.
2. Experimental section
2.1 Fabrication of multilayer $MoS_2$ phototransistor
First, a standard Remote Chemical Analysis (RCA) process was used to clean the heavily doped ${\rm p}^+$-Si wafers with a resistivity of 0.005 ${\mathrm \Omega} cm$ to remove organics, impurity and natural oxide on the surface of the wafers as the gate electrode. Then, a 6-nm HZO ferroelectric thin film was deposited by an ALD method at 200$^{\circ }$C, using ${\rm Hf}[{\rm N}({\rm C}_2{\rm H}_5){\rm CH}_3]_4$, ${\rm Zr}[{\rm N}({\rm C}_2{\rm H}_5){\rm CH}_3]_4$ and ${\rm H}_2{\rm O}$ as Hf, Zr and oxygen precursors, respectively. The HZO deposition consisted of 1-cycle ${\rm HfO}_2$ and 1-cycle ${\rm ZrO}_2$, with a deposition rate of 0.1 nm/cycle, which was repeated for 30 times to yield a HZO film with Hf:Zr = 1:1 and 6-nm thickness measured by ellipsometer. The HZO thin film was annealed at 550$^{\circ }$C for 60 s in a nitrogen atmosphere to form ferroelectric HZO, followed by a ALD-deposition of 2-nm ${\rm Al}_2{\rm O}_3$ as matching layer on the HZO thin film. The micro-mechanically exfoliated method was used to obtain the multilayer ${\rm MoS}_2$ from its bulk crystal, which, subsequently, was transferred onto the ${\rm Al}_2{\rm O}_3/{\rm HZO}/{\rm p}^+$-Si substrates. Next, the source and drain electrodes were prepared by electron beam lithography, followed by thermal evaporation of Cr/Au (15 nm/45 nm) and lift-off process. Finally, a rapid thermal annealing was performed at 300$^{\circ }$C for 180 s in a nitrogen atmosphere to improve the electrical contact between the metal electrodes and ${\rm MoS}_2$.
2.2 Sulfur treatment of $MoS_2$ flakes
After ${\rm MoS}_2$ flakes were transferred onto the ${\rm Al}_2{\rm O}_3/{\rm HZO}/{\rm p}^+$-Si substrates, they were placed at 10 cm upstream of 500-mg sulfur (S) source located in the center of the heating zone of a tubular heating furnace. The quartz tube is evacuated to 0.02 pa, and then the pump valve is closed and the ${\rm N}_2$ gas is flushed into the tube to raise air pressure to 1×105 pa. This process is repeated for three times to remove the residual air. Next, the furnace temperature rises to 200 $^{\circ }$C and keeps it constant for 60 min. During the S treatment, the pressure in the quartz tube is same as the atmospheric pressure, with a constant ${\rm N}_2$ flow rate of 18 sccm.
2.3 Measurement of electrical and optoelectronic properties
The transfer curve, output curve and response speed of transistor under dark state and light illumination are measured by Keysight B1500A semiconductor parameter analyzer at room temperature. A 528-nm laser is chosen as the light source for photoresponse measurement in light tight and electrically shielded ambient. The dark current is measured at threshold voltage ($V_{th}$= 0.21 V and 0.45 V for as-prepared and S-treated samples respectively). For photoresponse measurement, the sample is kept perpendicular to the incident light, and attenuators with different attenuation intensities are inserted in the middle between the light source and the sample to adjust the light intensity reaching the sample surface, where the light intensity is measured using a photometer.
3. Results and discussion
3.1 Structure of back-gate $MoS_2$ phototransistor
A back-gate ${\rm MoS}_2$ negative-capacitance field-effect phototransistor (NCFEPT) was fabricated, and its structural diagram and optical microscope photograph are shown in Figs. 1(a) and (b) respectively. The heavily doped ${\rm p}^+$-Si substrate was used as gate electrode, 6-nm HZO/2-nm ${\rm Al}_2{\rm O}_3$ were used as stack gate of ferroelectric/dielectric layers, multilayer ${\rm MoS}_2$ was used as active channel, and Cr/Au (15nm/45nm) were used as source and drain electrodes. The X-ray diffraction(XRD) results of HZO are shown in Supplement 1. Particularly, ${\rm Al}_2{\rm O}_3$ can act as both an insulator to reduce gate leakage and a passivation layer between HZO and ${\rm MoS}_2$ to obtain good interface characteristics [23,29]. Most importantly, ${\rm Al}_2{\rm O}_3$ can stabilize the NC effect as a capacitance matching layer and enhance device stability [30]. For better light absorption, multilayer instead of single-layer ${\rm MoS}_2$ with an uniformly light blue was chosen as channel [31] in this work, and its thickness measured by AFM (atomic force microscope) is 10.2 nm, as shown in Fig. 1(b). As a comparison, ${\rm Al}_2{\rm O}_3$/HZO was replaced by an 8-nm ${\rm Al}_2{\rm O}_3$, and a control sample of ${\rm MoS}_2$ field-effect phototransistor (FEPT) was prepared under the same processes.The cross-sectional high resolution transmission electron microscope (HR-TEM) imaging and the energy dispersive X-ray spectrometry (EDS) of Mo, S, Hf, Zr and Si elementals mapping of the back-gate ${\rm MoS}_2$ NCFEPT have shown in Fig. 1(c). Clear interfaces can be seen between different layers, and the EDS analysis demonstrates that all elementals are uniformly distributed and without inter-diffused. Figure 1(d) is an enlarged view of the crystal of HZO in the blue square and an image of fast Fourier transform (FFT) in the inset, and yellow dash line represent the crystallographic plane with a fixed interplanar crystal spacing of 0.294 nm, which is consistent with the previously reported orthogonal (111) crystal plane spacing ($d_{o(111)}$) of the HZO film [32,33]. In addition, the clear spots in the fast Fourier transform (FFT) image show a good crystallinity.
Fig. 1. (a) Structural diagram and (b) optical microscope photograph of the ${\rm MoS}_2$ phototransistor, where the inset is the AFM (atomic force microscope) imaging of the edge of ${\rm MoS}_2$ flake, the thickness is measured as 10.2 nm.(c) cross-sectional HR-TEM (high resolution transmission electron microscope) imaging of back-gate ${\rm MoS}_2$ NCFEPT, in which the inset picture is EDS(energy dispersive X-ray spectrometry) elemental mapping of Mo, S, Al, Hf, Zr and Si;(d) an enlarged view of the crystal of HZO film in the blue square and the fast Fourier transform image (inset).
3.2 Influence of NC effect on photoelectric properties of NCFEPT
Firstly, polarization intensity $vs.$ electric field ($P$-$E$) hysteresis loop and current density $vs.$ electric field ($J$-$E$) were measured on Au/HZO/TiN capacitor at 1 kHz to characterize the ferroelectricity of the HZO thin film, as shown in Fig. 2(a).Clear ferroelectric polarization characteristics with a fixed remnant polarization ($P_r$) of 12.4 $\mathrm{\mu} {\rm C}/{\rm{cm}}^2$ and a fixed coercive electric field ($E_c$) of 1.34 MV/cm are observed, and the $J$-$E$ loop also shows a significant polarization reversal current.
Fig. 2. (a) $P$-$E$ hysteresis loop and $J$-$E$ loop measured on Au/HZO/TiN capacitors, (b) $C$-$V$ curves measured on Au/(${\rm Al}_2{\rm O}_3$/HZO)/p-Si Metal-Oxide-Semiconductor (MOS) capacitors and (c) $C$-$V$ curves measured on Au/${\rm Al}_2{\rm O}_3$/p-Si MOS capacitors in a frequency range of 1 kHz to 1 MHz.
The capacitance-voltage ($C$-$V$) curves measured on Au/(${\rm Al}_2{\rm O}_3$/HZO)/p-Si MOS and Au/${\rm Al}_2{\rm O}_3$/p-Si MOS capacitors in a frequency range of 1 kHz to 1 MHz are shown in Fig. 2(b) and Fig. 2(c), respectively, for the purpose of depleting the carrier p-Si substrate was used. It can be seen that an obvious peak occurs in low frequencies below 30 kHz and the $C$-$V$ curves measured above 50 kHz are same as that of conventional MOS capacitor in Fig. 2(b), and no peak is abserved for the $C$-$V$ curves of control MOS capacitor in Fig. 2(c). Total capacitance ($C$) of the Au/(${\rm Al}_2{\rm O}_3$/HZO)/p-Si MOS capacitor is composed of MOS capacitance ($C_{MOS}$) and ferroelectric capacitance ($C_{Fe}$) in series. When the electric-field strength exceeds the coercive field of the HZO film, its ferroelectric polarity is reversed, and $C_{Fe}$ will become negative during the reversal. According to the formula of $C^{-1} = C_{MOS}^{-1} + C_{Fe}^{-1}$, the negative $C_{Fe}$ leads to a capacitance peak [34] at 0.25 V, which is a direct evidence of the NC effect [35]. In addition, the accumulation capacitance is larger for the Au/(${\rm Al}_2{\rm O}_3$/HZO)/p-Si MOS capacitor (150 pF) than the Au/${\rm Al}_2{\rm O}_3$/HZO/p-Si MOS capacitor (120 pF) under the same area, showing a higher $k$ value of gate stack for the former than the latter.
The electric properties of ${\rm MoS}_2$ NCFEPT with a stacked gate (${\rm Al}_2{\rm O}_3$/HZO) and ${\rm MoS}_2$ FEPT with only ${\rm Al}_2{\rm O}_3$ as gate dielectric are depicted in Fig. 3. It can be seen that both NCFEPT and FEPT have a high on-off current ratio ($I_{on}/I_{off}$) of more than $10^6$, in which $I_{on}/I_{off}$ of the NCFEPT is as high as $10^7$. The black lines in Fig. 3(a) present the transfer characteristics of the ${\rm MoS}_2$ FEPT, and its $SS$ in the forward and reverse sweeps is 85 mV/dec and 80 mV/dec, respectively. After inserting the HZO thin film in the gate stack, the $SS$ of the NCFEPT are lowered to 39 mV/dec and 45 mV/dec in the forward and reverse sweeps respectively. The $SS$ can be written as [18,19]:
where $C_S$, $C_{ox}$, and $C_{Fe}$ represent the barrier capacitance of ${\rm MoS}_2$, dielectric capacitance of ${\rm Al}_2{\rm O}_3$ and ferroelectric capacitance of HZO film, respectively. Obviously, in the condition of $C_{Fe} < 0$ and $|C_{Fe}|<|C_{ox}|$, $SS$ will break the limit of 60 mV/dec owing to the NC effect.In particular, when $V_{gs}$ of NCFEPT sweeps from negative voltage to positive voltage, the polarization direction of HZO film changes from the downward to the upward, so that the channel electrons are firstly depleted by the downward ferroelectric polarization to turned off the phototransistor, and then, when $V_{gs}$ exceeds $V_{th}$,the polarization direction is reversed, and the upward ferroelectric polarization will enhance the accumulation of electrons in the channel, which causes a drastic increase of the channel current, resulting in a steep subthreshold slope (small $SS$) and high $I_{on}$/$I_{off}$ ratio. Fig. 3(b) shows the output characteristic curves of both NCFEPT (solid lines) and FEPT (dashed lines) when $V_{gs}$ changes from 0 V to 2 V. Clearly, the output current of NCFEPT is increased by a factor of 347% as compared with that of the FEPT for $V_{gs}$ = 2 V and $V_{ds}$ = 2 V.Fig. 3. (a) Transfer curves and (b) output curves of ${\rm MoS}_2$ NCFEPT and FEPT phototransistors under different $V_{gs}$ from 0 V to 2 V with a step of 0.5V.
The transfer and output characteristic curves of the ${\rm MoS}_2$ NCFEPT and FEPT in the dark and under the illumination of various optical powers ($P$) are shown in Fig. 4, with a fixed $V_{ds}$ of 0.4 V for the $I_{ds}$-$V_{gs}$ measurement and a fixed $V_{gs}$ of 0.21 V ($V_{th}$ of NCFEPT) or 0.3 V ($V_{th}$ of FEPT) for $I_{ds}$-$V_{ds}$ measurement, where the dark-state current was measured before illumination at a wavelength of 528 nm. As shown in Fig. 4(b), the $SS$ values of ${\rm MoS}_2$ NCFEPT and FEPT are increased from 39 mV/dec and 62 mV/dec at optical powers of 2.32 $\mathrm{\mu} {\rm W}/cm^2$ to 79 mV/dec and 120 mV/dec at optical powers of 173 $\mathrm{\mu} {\rm W}/cm^2$ respectively. Obviously, the NC effect generated by the HZO ferroelectric thin film can maintain a lower $SS$ for the NCFEPT than the FEPT under different light intensities. For ${\rm MoS}_2$ phototransistors operating in the subthreshold region, this excellent subthreshold behavior lays a foundation for more sensitive light detection as compared to conventional photodetectors.
Fig. 4. (a) Transfer characteristics at a fixed $V_{ds}$ of 0.4V and (b) output characteristics at a fixed $V_{gs}$ of 0.21V of the ${\rm MoS}_2$ NCFEPT under different illuminations; the corresponding characteristics are plotted in (c) and (d) for ${\rm MoS}_2$ FEPT respectively.
Two important parameters of the responsivity ($R=I_{ph}\cdot P_{eff}^{-1}$) and the detectivity ($D^*=RA^{1/2} (2eI_{dark}^{-1/2}$) can be extracted from the $I_{ds}$-$V_{ds}$ curves in Figs. 4(b) and (d). It is worth noting that photogenerated current ($I_{ph}$) and the incident power ($P_{eff}$) are calculated as $I_{ph} = I_{light} - I_{dark}$ and $P_{eff}=AP$, where $I_{dark}$ and $I_{light}$ are the $I_{ds}$’s in dark and under illumination respectively, $A$ is area of the ${\rm MoS}_2$ channel ( 20 $\mathrm{\mu} {\rm m}^2$) and $P$ is the light intensity at the sample surface measured by a photometer. The $R$ represents a photogenerated current that a photodetector can produce at unit incident power, and the $D^*$ can compare the light detection performance of different devices. It can be seen that the maximum responsivity ($R_{max}$) and detectivity ($D^*_{max}$) of the NCFEPT are increased to 579.37 A/W and 3.736×1014 cmHz1/2W−1 at a light intensity of 2.32 $\mathrm{\mu} {\rm W}/cm^2$ , as compared with 177.15 A/W and 4.945×1013 cmHz1/2W−1 of the FEPT respectively. The obvious improvements in optoelectronic performance are from the lower $SS$ and the reduction of the ${\rm MoS}_2$ channel barrier caused by the NC effect. Particularly, a fixed $V_{gs}$ of 0.21V is set to maintain the phototransistor off to suppress the $I_{dark}$. Under light illumination, the photogating effect of ${\rm MoS}_2$ leads to a change of $V_{th}$($\Delta V_{th}$) as well as a reduction of the energy barrier($q\Delta V$) in the channel, and oppositely, the voltage amplification effect of negative capacitance can make $\Delta V$ greater than $\Delta V_{th}$, and thus the channel barrier can be lowered to break through the limit of $\Delta V_{th}$, so that more electrons can get over the energy barrier and form photocurrent [20].
3.3 Influence of defects engineering on photoelectric properties of NCFEPT
Further, to investigate the effects of defect states (mainly S vacancies) in ${\rm MoS}_2$ on the optoelectronic properties, a sulfur (S) treatment of ${\rm MoS}_2$ was carried out at 200 $^{\circ }$C in ${\rm N}_2$ ambient in a CVD system (see "Experimental section" for details). Obviously, after the S treatment, the $SS$ of the transistor are lowered to 33 mV/dec and 35 mV/dec in the forward and reverse sweeps, respectively, as shown by the red lines in Fig. 5(a). which can be attributed to a cleaner interface and weaker Fermi pinning effect after S treatment [36]. Changes of more electrical parameters after the vulcanization, e.g. $V_{th}$, mobility ($\mu$) and hysteresis, are listed in Table 1. A visible change in $V_{th}$ can be seen after the S treatment as compared with its counterpart without S treatment, and the $V_{th}$ is shifted from pre-treatment 0.21V to post-treatment 0.45V, which can be attributed to the reduction of the S vacancies in ${\rm MoS}_2$ after S treatment [37]. The $\mu$ is enhanced from pre-treatment 24.51 cm2V−1s−1 to post-treatment 32.44 cm2V−1s−1, owing to decreased scattering of defects after the SVs are repaired. And the slight reduction in hysteresis is also from the repair of the SVs.
Fig. 5. Transfer curves of ${\rm MoS}_2$ NCFEPT and S-NCFEPT (a) in semi-logarithmic coordinates and (b) in linear coordinates; (c)PL spectroscopy, (d)Raman E2g and A1g characteristic peak before and after S treatment on ${\rm MoS}_2$ flake ; (e) PL intensity at 1.82 eV across the entire surface of ${\rm MoS}_2$ flake.
Table 1. Electrical parameters of phototransistors before and after vulcanization
Also, Raman and photoluminescence (PL) spectroscopies were measured on ${\rm MoS}_2$ before and after sulfur treatment, as shown in Figs. 5(c) and (d) respectively. The S vacancies in ${\rm MoS}_2$ exhibit the nature of donors, which is one of the main external factors that ${\rm MoS}_2$ naturally behaves as N-type [38]. During the S treatment, the repair of the S vacancies is equivalent to the P-type doping effect, which makes the Raman $A1g$ characteristic peak blue shift [39] (from 407.42 ${\rm cm}^{-1}$ to 408.51 ${\rm cm}^{-1}$). Furthermore, the PL intensity is weakened after S treatment, which is another evidence of the SVs reduction [37,39]. To exclude accidental drop of the PL intensity in ${\rm MoS}_2$, the entire sample surface was scanned and the PL intensity at 1.82 eV was selected to plot Fig. 5(e). It can be seen that the PL brightness across the surface of the ${\rm MoS}_2$ flake is weaker for the S-treated ${\rm MoS}_2$ than the as-prepared ${\rm MoS}_2$, strongly supporting the repair of the SVs. Additionally, Supplement 1 shows the PL brightness of ${\rm MoS}_2$ flake without Au electrode covered.
The transfer and output characteristic curves of the phototransistor under dark and light conditions after S treatment are shown in Figs. 6(a) and (b), with a fixed $V_{ds}$ of 0.4 V for the transfer-curve measurement and the $V_{th}$ of 0.44 V selected for dark-state current measurement. From Fig. 4(c) and Fig. 6(a), the |$\Delta V_{th}$| before and after S treatment can be extracted to be 0.56 V (from $V_{th,dark}$ = 0.217 V to $V_{th,light}$ = -0.343 V at a light intensity of 177$\mathrm{\mu} {\rm W}/cm^2$) and 0.45V (from $V_{th,dark}$ = 0.44V to $V_{th,light}$ = -0.01V at a light intensity of 177$\mathrm{\mu} {\rm W}/cm^2$), respectively. A clear reduction in |$\Delta V_{th}$| and a lower $SS$ can be observed after S treatment, where the reduction in $|\Delta V_{th}|$ comes from the passivation effect of S treatment on SVs in ${\rm MoS}_2$ and lower $SS$ is attributed to the reduced defects and interface trap densities within the device channel and at the interface [36]. Figs. 6(c) and (d) show the response time under 528-nm green light. The response time of ${\rm MoS}_2$ FEPT and NCFEPT is not much different, and the rise time ($\tau _r$) and decay time ($\tau _f$) of ${\rm MoS}_2$ NCFEPT are measured to be 18 ms and 24 ms, respectively. However, the $\tau _r$ and $\tau _f$ of the phototransistor after the S treatment is significantly reduced to 3 ms and 4 ms.
Fig. 6. (a) Transfer characteristics, (b) output characteristics at a fixed $V_{gs}$ of 0.21V and (c) response time of the ${\rm MoS}_2$ NCFEPT with sulfur treatment under different illuminations; (d) response time of the MoS2 FEPT and NCFEPT without sulfur treatment.
When light reaches the ${\rm MoS}_2$ surface, photogenerated carriers will be generated. Since holes are trapped by defect states at the band edge of ${\rm MoS}_2$, the probability of the hole-electron recombination will be reduced, and the lifetime of the photogenerated electrons will be increased, thereby improving photocurrent gain [31,40]. However, at the same time, due to the participation of the defect states in the photoconductive process, the photoelectric response speed of the device is reduced. Therefore, regulating the defect-state density in 2D materials is of great significance to the photoelectric performance. As shown in Table 2, the $R_{max}$ and $D^*_{max}$ of the phototransistor after S treatment are 423.43 A/W and 1.329×1014 cmHz1/2W−1 at a light intensity of 2.32 $\mathrm{\mu} {\rm W}/cm^2$ extracted from Fig. 6(b). Although weakening of the photogating effect reduces the final $I_{ph}$ of the device, a large responsivity over 400 A/W and an excellent detectivity with an order of ${\rm 10}^{14}$ magnitude and the significantly improved response speed are collectively achieved for the S-treated sample.
Table 2. Optical performance parameters of MoS2 FEPT, NCFEPT and S-NCFEPT
Figure 7 compares optoelectronic properties of ${\rm MoS}_2$ phototransistors between the reported work in recent years and this work. Obviously, owing to the low $SS$ and reduced channel barrier caused by the NC effect, excellent responsivity and detectivity of ${\rm MoS}_2$ NCFEPTs before and after S treatment are demonstrated in this work, and also the response speed is significantly improved after S treatment, attributed to the repair of the SVs in ${\rm MoS}_2$ by defect engineering, which leads to the reduction of defect- trapping photogenerated holes, thus improving the response speed.
Fig. 7. Comparison of optoelectronic properties of ${\rm MoS}_2$ phototransistors between the reported work [41–49] and this work: (a) detectivity $vs$. respond time and (b) detectivity $vs$. responsivity.
4. Conclusions
In summary, a NC-${\rm MoS}_2$ phototransistors with HZO ferroelectric layer and ${\rm Al}_2{\rm O}_3$ matching layer as gate stack has been fabricated. Owing to the negative-capacitance effect of the ferroelectric material, the transistor has achieved a $SS$ as low as 39 mV/dec and an ultrahigh detectivity of 3.736×1014cmHz1/2W−1 under an incident light with a wavelength of 523 nm and $P$ = 2.32 $\mathrm{\mu} {\rm W}/{\rm cm}^2$ at room temperature. Further, through the S treatment on ${\rm MoS}_2$ channel, the electrical properties of devices are significantly improved, in which the $SS$ is lowered from 39 mV/dec to 33 mV/dec, and the mobility of ${\rm MoS}_2$ is increased from 24.5 cm2V−1s−1 to 32.4 cm2V−1s−1. As a crucial factor in photodetector performance, the response time of the S-treated sample for light detection is greatly dropped from 18-24 ms to 3-4 ms and a large responsivity over 400 A/W with an order of ${\rm 10}^{14}$ magnitude at the incident power of $P_{eff}$ = 46.4 pW are achieved. Therefore, combination of the NC effect from the ferroelectric thin film and the control of the defect states in ${\rm MoS}_2$ by passivation of the S vacancies provides a promising way to optimize strategy of ${\rm MoS}_2$ photodetector.
Funding
National Natural Science Foundation of China (61774064, 61851406, 61974048); National Key Research and Development Program of China (2018YFB2200500).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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